The present invention relates to a process for producing a hydroxyalkyl monoacrylate, and more particularly to an improved and industrially advantageous process for the production of a hydroxyalkyl monoacrylate, which is capable of effectively recovering a reaction catalyst for reuse thereof.
Hydroxyalkyl monoacrylates are compounds represented by the general formula of CH.sub.2 .dbd.CHCOO(CH.sub.2).sub.n OH, wherein n is an integer of 1 to 10, and are useful as raw materials for polymers or copolymers having practically interested properties such as appropriate flexibility and hydrophilicity, because these compounds contain both hydrophobic alkyl units and hydrophilic hydroxyl groups in their molecules. In addition, it has been expected that the hydroxyalkyl monoacrylates are used in the form of crosslinkable polymers or copolymers in paint applications since hydroxyl groups thereof have a high reactivity.
It is known in the art that the hydroxyalkyl monoacrylates are produced by reacting acrylic acid or its derivative with alkane diol. For example, in German Patent No. 15118572, there is described a method of replacing a terminal group of alkane diol with an acrylic group by using a strong protonic acid such as sulfuric acid or hydrochloric acid as a reaction catalyst.
In addition, titanium alkoxides have been extensively used as catalysts for esterification or transesterification, and have also exhibited a high activity in the afore-mentioned reaction between acrylic acid or its derivative and alkane diol, and a high selectivity for the aimed product. In Japanese Patent Application Laid-open (KOKAI) No. 52-153913(1977), there have been proposed a method of using a stannoxane compound as a reaction catalyst upon producing dimethylaminoethyl methacrylate by transesterification reaction. In Japanese Patent Publication (KOKOKU) No. 5-76350(1993), there have been proposed a method of using a halogen-substituted distannoxane-based catalyst as a high activity esterification catalyst. Further, in Japanese Patent Publication (KOKOKU) No. 46-39848(1971), there have been proposed a method of using dibutyl-n-tin oxide as a transesterification catalyst, and in Japanese Patent Application Laid-open (KOKAI) No. 7-97387(1995), there have been proposed a method of recovering a distannoxane catalyst from a mixed solution containing the distannoxane catalyst and saccharose-6-ester by extraction.
However, in the method described in German Patent No. 15118572, there arise problems such as corrosion of a reactor used due to existence of the strong acid catalyst, and decrease in selectivity for aimed hydroxyalkyl monoacrylate due to cyclization of the raw alkane diol and formation of high-boiling reaction products. In addition, the above-mentioned method inevitably requires complicated water-washing and neutralizing steps for separating the reaction catalyst from the reaction product.
In the method using the titanium alkoxide catalyst, although the titanium alkoxide catalyst shows a high catalytic activity and a high selectivity for the aimed product, there also arises such a problem that since the catalyst is highly hydrolyzable, the catalytic activity thereof is abruptly lost even in the presence of a very small amount of water. Therefore, when such a titanium alkoxide catalyst is used, it is required to completely remove water from the reaction system. Further, in the case of using the dibutyl-n-tin oxide catalyst, since the catalyst has a low solubility in a reaction substrate, it takes a long period of time until the catalytic activity thereof is sufficiently exhibited.
In the methods described in Japanese Patent Application Laid-open (KOKAI) No. 52-153913(1977) and Japanese Patent Publication (KOKOKU) No. 5-76350(1993), although there has been used the stannoxane catalyst which is excellent in catalytic activity, etc., in these publications, there are neither descriptions nor teaching concerning recovery and reuse of the catalyst, which are important when it is intended to industrially put these methods into practice.
More specifically, in Japanese Patent Application Laid-open (KOKAI) No. 52-153913(1977), although it is merely described that the reaction solution is distilled under reduced pressure to distill off the methacrylate as a reaction product, there are neither descriptions nor teachings concerning recovery and reuse of the stannoxane catalyst. In addition, in Japanese Patent Publication (KOKOKU) No. 5-76350(1993), it is only described that the esterification catalyst is recovered from the distillation residue or the like and can be reused as it is.
Also, in Japanese Patent Application Laid-open (KOKAI) No. 7-97387(1995), there is only described a method of separating the catalyst upon the production of relatively stable esters such as saccharose-6-ester.
Meanwhile, according to the present inventors' knowledge as to the production of hydroxyalkyl monoacrylate using the stannoxane catalyst, it would be difficult to actually conduct the distillation and separation processes under a high temperature condition for recovering the catalyst. More specifically, in the case where the acrylic acid derivative or methacrylic acid derivative exists in the reaction system as a starting material or a reaction product, there arises a problem that the derivative not a little undergoes thermal polymerization or chemical deterioration when heated in the presence of the stannoxane catalyst. It is considered that such a problem is caused due to the fact that the hydroxyalkyl monoacrylate shows an extremely low thermal stability and an extremely high reactivity in the presence of the stannoxane catalyst.
Especially, in the case where alkyl diacrylate having a high unsaturated bond content always exists as an unreacted raw material or a by-product in the reaction solution together with an acid catalyst such as stannoxane-based catalyst in the production of the hydroxyalkyl monoacrylate by subjecting acrylic acid or alkyl diacrylate and alkane diol to esterification and/or transesterification, the above-mentioned problem becomes more remarkable.
As a result of the present inventors' earnest studies, it has been found that in the production of the hydroxyalkyl monoacrylate comprising subjecting acrylic acid or an acrylic acid derivative and alkane diol to esterification and/or transesterification, by using a specific stannoxane catalyst and recovering the catalyst by extraction, the above-mentioned problems can be readily overcome. The present invention has been attained on the basis of the finding.